How is adjectival clause related to parts of speech?
1 adjectival clause functions as an adverb.
2 adjectival clause functions as an adjective.
3 adjectival clause functions as a verb.
4 adjectival clause functions as a noun.

The answer to your question would be that adjectival clauses function as adjectives. That is, your answer would be option 2.

An adjectival clause is a dependent clause that, just as adjectives, modifies a noun or a pronoun. They provide information that is consired essential or non-essential to the meaning of the sentece. The former are called restrictive adjective clauses and the latter non-restrictive adjective clauses.

2

Explanation:

Adjectival clauses are dependent clauses that are joined to a noun or pronoun in order to describe it. This would make them work as adjectives. They are usually introduced by a relative pronoun (like who, which, whose, etc)

example:

This sentence, which I am writing, is an example of an adjectival (or adjective) clause